DPD systems are often employed in communications system transmitters to allow a power amplifier to be driven into its non-linear region. In a typical DPD prior art system, a baseband signal is processed by a predistorter. The predistorter is a nonlinear device which creates a signal that is forwarded to the nonlinear power amplifier, such that the cascade of the nonlinear predistorter and the nonlinear power amplifier produces a system that is linear overall. Thus, the signal being sent from the nonlinear device to an antenna is simply a scaled up version of the signal that arrived from the baseband.
The baseband signal comes in and is sent through a total of Npdf predistortion functions. Power amplifiers typically have different characteristics at different transmission power levels. Hence, different predistortion functions are typically needed to correct power amplifier performance at different transmission power levels. The number of predistortion functions (Npdf) depends on the power amplifier being used because different power amplifiers require a different number of predistortion functions. Each predistortion function is responsible for linearizing a power amplifier when the power amplifier is transmitting inside a specific transmission power range. For every possible average transmission power, there may be only a single predistortion function responsible for linearization.
The particular predistortion function that is used is determined by the actions of a power estimator, a slicer, and a multiplexer. The power estimator produces estimates of the baseband signal power or, in effect, the power amplifier's transmit power. The power estimates are forwarded to the slicer which examines the power estimate and determines which predistortion function is responsible for linearizing the power amplifier at that particular transmission power level. The input to the slicer is the current estimated power value and the output is a number between 1 and Npdf indicating which predistortion function should be used to predistort the power amplifier.
Often the characteristics of the power amplifier change with time and it is necessary to periodically update the characteristics of the predistorter. These periodic updates are under the control of a predistortion model calculator which first sends a signal to a capture buffer indicating that it should capture and record the data going into the power amplifier and the data coming out of the power amplifier. The capture buffer is rather simple and upon the receipt of a trigger signal, it begins blindly recording samples into its memory buffer until it has recorded the appropriate number of samples. The amount of data to record will vary based on different types and models of power amplifiers, but will typically be in the range of several thousand samples. After the capture buffer has finished capturing data, the predistortion model calculator reads the data from the memory buffer and then processes the data so as to update the predistortion functions.
The predistortion function that is updated is chosen based on the average power of the signal coming out of the power amplifier. The predistortion function which will be updated is the predistortion function which is responsible for linearizing the power amplifier at the power level that was measured on the output of the power amplifier.
A significant limitation with the prior art is that the trigger affects the capture of a random set of data. The trigger signal arrives at a random moment in time without any regard to the signal that is currently being sent through the power amplifier. For example, a power amplifier can often be linearized quite well when it is transmitting at a fixed average transmission power level for some time. If the transmission power level changes, after some time, it will also be possible to linearize the power amplifier at the new average transmission power level. However, during a change in transmission power from one level to another, there will be some time when the power amplifier's characteristics are transitioning from one power level to another. During these transient periods, the power amplifier behaves somewhat unpredictably and is rather difficult to linearize. Because of the random timing of the trigger signal, it is possible that the capture buffer will capture data during one of these transient periods. If data from one of these transient periods is used to update a predistortion function, that predistortion function will only be able to linearize data during a transition period. Therefore, if the power amplifier transmits data at a fixed transmission power, this predistortion function will not be able to linearize the power amplifier and poor performance will result.
For the purposes of further explanation, suppose that a power amplifier has been transmitting at a transmission power level of Pmax for a long period of time. Further suppose that the transmission power changes to Pmax-5 dB and that this particular power amplifier takes 1 ms to reach its new steady state. If data is captured after the transmission power has been reduced, but before the power amplifier has reached its new steady state, the captured data would describe the performance of the power amplifier during this transient state and cannot be used to create a predistortion function. It would therefore be beneficial if a mechanism existed that could prevent the capture buffer from capturing transient data.
A further discrepancy with the prior art described above becomes apparent in a situation where the transmission power level may be stable for long periods of time, but the transmission power level can change abruptly for short periods of time. For example, in a lightly loaded WCDMA (Wideband Code Division Multiple Access) transmitter that supports a High Speed Downlink Power Amplifier (HSDPA), it is a regular occurrence that the power amplifier may transmit at a transmission power level of Pmax-10 dB for an extended period of time. If a mobile station is scheduled to receive a downlink transmission that is rather short in length, it is possible for the transmission power to suddenly jump to Pmax for a short amount of time (perhaps 2 ms) and then drop back down to Pmax-10 dB after the transmission to the mobile station has been completed.
With a random trigger, the probability of capturing data while the power amplifier is transmitting at Pmax is very low. For example, if these 2 ms transmissions only appear once per second, the probability of capturing data while the power amplifier is transmitting at Pmax is only about 1 in 500. This is a problem because the predistortion functions need to be updated regularly as the power amplifier's characteristics change. If the predistortion function that is used at a transmission power of Pmax is not updated regularly, the power amplifier may slowly lose performance at that transmission power level resulting in a decreased capacity and increased adjacent channel interference.
Therefore, it would also be beneficial if a mechanism existed that would ensure that predistortion functions that have not been trained in a long time be properly updated, even if data to update these predistortion functions is difficult to find.